The present invention relates generally to heaters or furnaces and, more particularly, to a fuel-input modulated furnace with improved thermal efficiency.
Contemporary heating systems, for example, furnaces for space heating or make-up air heating, are generally equipped with fuel valves which can be used to modulate the fuel-input rate to the heater in order to maintain a stable and controlled temperature. This type of system generally has a limited range of fuel-input modulation. In addition, these contemporary heaters suffer from a loss of thermal efficiency as the fuel-input rate is reduced below their full fuel-input rate.
Alternatively, such controlled temperature systems may also include some type of damper to open or close a combustion air by-pass. As a result of the ability to vary the amount of combustion air accessible to the system, these systems offer a broader possible range of fuel input modulation. Typically, however, the damper has only a few static positions available, which limits the actual control over the ratio of fuel to air.
Other known heating systems provide improved thermal efficiency by controlling the supply of fuel and combustion air in predetermined incremented amounts. However, these systems are complex and costly, requiring accurate sensor systems, flow control devices such as mechanical jackshafts, and the application of algorithms and control units to regulate the thermal efficiency.
There is a need, therefore, for a system and method for improving the thermal efficiency of a heating system by controlling the quantity of combustion air in response to the fuel-input rate modulation.